Its easy to armchair quarterback after the fact.
Heavy, vault like doors are not practical on an aircraft for the same reason you dont want your mustang to weigh in at 4000 lbs.
Armed guards? So how hard would it be for a couple terrorists to kill an armed guard or two?
The pilot being the only one to help the passengers in the air is absurd comment. The crew is a few people. The passengers have to defend themselves! If two terrorists are threatening you with knives, and taking over the aircraft, what do you do? Hide in the bathroom and make cell phone calls? Especially after it becomes apparant that they intend to fly these planes into the ground? No, you take action. 50 passengers can certainly take over two terrorists, but making it happen is understandably easier said than done.
Even heavy doors can be broken down. Its not a bank, its an aircraft.
There is no simple solution to this problem, and something we will have to live with if we want to have air travel.
------------------
1993 GT/AOD
'93 Mustang GT